Soak rolls in water. Clean and wash the carrots, dice 1 carrot finely and cut the rest into slices. Peel potatoes, wash thoroughly and cut into quarters. Boil the potatoes and carrot slices in salted water, cover and cook for about 25 minutes.
Meanwhile peel the onions, finely dice one onion and slice the remaining onions. Knead the pressed bread roll, minced meat, diced onion, egg, mustard, salt and pepper with the dough hooks of the hand mixer.
Form 8-12 meatballs from the minced meat mass depending on size. Heat the oil in a large pan. Fry the meatballs for 4-5 minutes on each side at medium heat. Add the onion rings shortly before the end of the frying time.
In the meantime wash parsley, dab dry and chop finely. Put carrot cubes in boiling salted water and cook for about 4 minutes, pour into a sieve, drain. Heat milk and butter in a small saucepan.
Drain the potato and carrot slices. Add hot milk and mash with a potato masher. Sprinkle the mashed potatoes and carrots with carrot cubes. Serve the meatballs and onion rings on four plates sprinkled with parsley.
Garnish as desired with parsley leaf.